Austenitic stainless steel cannot be hardened through heat treatment. The purpose of heat treatment on these alloys is to remove the cold work hardening effect, dissolve harmful secondary phases again, and reduce residual stress to an acceptable level. Heat treatment can also produce recrystallized structures with smaller grain sizes in cold-worked stainless steel.
Solution annealing can soften materials after cold working and dissolve secondary phases that may precipitate during hot working or welding processes. The term "complete annealing" usually refers to the material being in its optimal metallurgical state, with complete dissolution of the secondary phase and complete homogenization of the metallographic structure. Completely annealed stainless steel has the best corrosion resistance and ductility. Due to the fact that solid solution annealing is carried out at high temperatures, annealing in an air environment can generate oxide scales on the surface, which must be removed by descaling or pickling to restore the surface's corrosion resistance.
prepare
Before annealing, it is necessary to remove surface grease, oil, cutting fluid, forming lubricant, colored pen markings, and other pollutants. Annealing can cause pollutants to "burn" into the surface and must be ground, otherwise it is difficult to remove. The infiltration of carbon containing pollutants into the surface can cause carbonization or sensitization, which can easily lead to intergranular corrosion during use. Therefore, surface cleaning before heat treatment is crucial for ensuring product quality. Cleaning methods include soaking or spraying chemical reagents. The cleaning agents used for stainless steel degreasing include hot alkaline solutions and chemical solvents.
Low melting point metals such as lead, copper, and zinc must be avoided from contaminating the surface. During annealing, they can cause grain boundary infiltration, leading to so-called liquid metal embrittlement and intergranular cracking. Therefore, before high-temperature treatment such as annealing and welding, it is necessary to clean up the residual pollutants on the surface.
temperature
The minimum annealing temperature refers to the lowest temperature at which the microstructure homogenizes and dissolves carbides and intermetallic precipitates. To ensure complete dissolution of precipitates and restore corrosion resistance, the annealing temperature must be higher than this temperature. The upper limit of annealing temperature is based on no warping, avoiding excessive grain growth, and minimizing the number of difficult to clean oxide scales as much as possible. The following table lists the minimum annealing temperatures for some austenitic stainless steels. High performance austenitic stainless steel requires homogenization of its microstructure at high temperatures, so their solution annealing temperature is higher than that of standard austenitic stainless steel.
annealing time
Maintaining the solution annealing temperature for 2-3 minutes is sufficient to dissolve a small amount of carbides and other secondary phases, and can also soften the cold formed material. During solution annealing, in order to ensure that the workpiece reaches the solution annealing temperature from the outside to the inside, the insulation time is usually 2-3 minutes per millimeter of thickness. If the amount of precipitates is large, especially with χ and σ When in phase, it is necessary to extend the insulation time.
If the solution annealing time is too long or the temperature is too high, a large amount of oxide skin will be generated, making cleaning difficult and costly. Long term annealing also increases the possibility of unqualified dimensional deformation during the heat treatment process. High molybdenum and high-performance austenitic stainless steel rapidly forms oxide scales in a naturally ventilated furnace. Molybdenum trioxide usually evaporates and leaves the surface as a gas. If volatilization is inhibited, liquid molybdenum trioxide will accumulate on the surface, accelerating the oxidation process. This is what is called "intense oxidation". The measures to minimize oxidation of high molybdenum steel include:
• Avoid conditions that inhibit volatilization (filling too tightly and sealing the furnace too tightly);
• Materials with severe oxide scale cannot be re annealed;
• Avoid prolonged exposure to environments above the minimum annealing temperature;
• Use the lowest annealing temperature that can be operated;
• Use a protective atmosphere.
atmosphere
Air and oxidizing combustion gases form the most economical and effective annealing atmosphere for stainless steel. However, the oxide skin generated by air annealing must be removed to restore corrosion resistance. Protective atmospheres such as argon, helium, hydrogen, cracked ammonia, hydrogen/nitrogen mixture, and vacuum can reduce the formation of oxide scales, but the cost is relatively high. Bright annealing is generally carried out in hydrogen or cracked ammonia gas with a dew point of -40 ° C or lower. Under normal operating conditions, annealing in a protective atmosphere will not produce visible oxide skin, so there is no need to clean it after annealing.
cooling
To prevent the precipitation of chromium carbide or other intermetallic phases, austenitic stainless steel may require rapid cooling after annealing. The need for rapid cooling and the choice of cooling method depend on the cross-sectional size and grade.
In the vast majority of cases, 304L and 316L with thin sections will not precipitate harmful phases after air cooling. As the cross-sectional size, carbon content, and alloy content increase, the necessity for rapid cooling also increases. High performance austenitic stainless steel requires rapid cooling regardless of thickness. Common cooling methods include forced air cooling, water spray cooling, or water quenching cooling. After vacuum annealing, inert gas quenching will not produce oxide skin.
If the annealed material still needs to undergo hot processing such as welding, it is best to perform maximum cooling such as water quenching after annealing. This can make the material better resistant to the adverse effects generated by the subsequent thermal cycles. When selecting cooling methods, possible deformation and new residual stresses should be considered.
Cleaning after annealing
Due to the high chromium content in the heat treated oxide skin, the chromium content of the metal adjacent to the oxide skin is reduced, resulting in a decrease in corrosion resistance. To fully restore corrosion resistance, it is necessary to remove the oxide skin and poor chromium metal layer.
The most commonly used cleaning method is shot peening to remove oxide scale, followed by acid washing to remove poor chromium metal. The most common method for pickling stainless steel is immersion pickling, which can also be performed by spraying, gel and ointment.
The acid used for pickling is very harmful and must be used in accordance with safety regulations (ventilation, wearing goggles and gloves, wearing safety clothing, etc.). The workpiece after pickling must be neutralized and thoroughly rinsed with a large amount of clean low chlorine water. Collect and dispose of waste liquid separately according to local hazardous waste management regulations.